Abstract: A method and apparatus for reducing noise originating in a flowing stream of gases in which the noise is progressively absorbed from the flowing stream to provide noise of reduced intensity relative to noise intensity at the upstream end of the stream, and the noise of reduced intensity actively attenuated with controlled sound energy. To facilitate the active attenuation of noise, the flowing stream is preferably divided into a plurality of smaller streams after a level of reduced intensities is attained and the active attenuation performed independently in each of the smaller streams. The apparatus is in the nature of a conduit (10) which defines a single duct extending from the inlet end (14) for a portion of the length of the conduit. Downstream, the single duct joins with a plurality of ducts (32,34) extending to the outlet end (16). An active noise attenuating device is preferably provided in each of the plurality of ducts.

Abstract: A decoupling ring provided on the front annular face of the nose shell of a craft, surrounding the sonar array. This ring decouples the vibration in the nose shell from the fluid path. The decoupling ring is constructed of one or more mass elements on one or more compliance elements. The individual compliance elements are preferably vibrationally speaking springs and the mass elements are preferably metal segments which form a mass-spring system. The dimensions and characteristics of the mass and compliance rings are chosen to have a fundamental resonant frequency well below the sonar frequency range of interest, thus acting as a low pass filter. When the vibrational energy has a frequency above the resonant frequency of the mass spring system, the vibrational energy is attenuated effectively. The low frequency resonance of the decoupling ring is obtained by using a large mass and a large compliance in the mass and compliance elements, respectively.

Abstract: An array plate for use in an underwater craft. The craft having an array of sonar elements which has a selected range of operating frequencies. The array plate having at least one layer of material in which the layer(s) of material are designed to have selected natural frequencies of vibration throughout the range of sonar operating frequencies. The natural frequencies resulting in standing waves having selected wavelengths that develop along the layer(s) of material. The sonar elements are mounted upon the layer(s) of material such that adjacent sonar elements are spaced apart a distance of one half the average wavelength of the standing waves.